The present invention relates to a process for absorbing nitrogen oxides from gas mixtures containing said oxides. The process according to the present invention is particularly, but not exclusively, useful for removing nitrogen oxides contained in gases produced by combustion processes such as, for example, the gas emissions of thermoelectric power stations, kilns and furnaces for producing cement and ceramic materials, motor vehicle exhausts, and domestic heating system exhausts. These gases are also produced by various specific industrial processes, such as the production of semiconducting materials.
Absorption of nitrogen oxides by Baxe2x80x94Cuxe2x80x94O mixed oxides has been described in the literature (Masato Machida et al., xe2x80x9cNO Removal by Absorption into BaOxe2x80x94CuO Binary Oxidesxe2x80x9d, J Chem Soc Chem Commun, 1990, p. 1165; M. Machida et al., xe2x80x9cCatalytically Accelerated Solid Gas Reaction Between NO and Baxe2x80x94Cuxe2x80x94O for Efficient NO Removalxe2x80x9d, Proceedings of the 10th International Congress on Catalysts, Jul. 9-24 1992, Budapest, Hungary).
In particular, the activity of reversible absorption of NO of two phases of mixed oxide, BaCuO2.1 and BaCuO2.5, has been noted.
The main drawback in using BaCuOx compounds where x=2.1 and x=2.5 is the reactivity of these compounds in the presence of CO2 and H2O, which are unavoidable components of combustion exhaust gases; this reactivity leads to the rapid and irreversible passivation of the nitrogen oxide absorption activity.
Absorption of NOx by materials based on BaCuO2, such as for example MnO2BaCuO2, is also known (EP 540280 and EP 580389).
The addition of MnO2, reported as a solution to the above described drawback, has proved itself ineffective, contrary to what has been claimed, as demonstrated by our experiments.
Moreover, WO97/28884 describes the NO and NO2 absorption activity of compounds having the formula Ba2Cu3O6xc2x1d, where d has a value between 0 and 1, for example (Ba2xe2x88x92xAx)Cu3O6xc2x1d; the same document describes the absorption of nitrogen oxides by (Ba2xe2x88x92xAx)Cu3O6xc2x1d where A is an alkaline or alkaline-earth metal or a lanthanide, for example a compound having the formula e (Ba2xe2x88x92xSrx)Cu3O6xc2x1d.
Ba2Cu3O6xc2x1d compounds absorb nitrogen oxides reversibly and, if synthesized appropriately, are resistant to the action of CO2 and H2O; however, the regeneration temperature is no less than 650xc2x0 C. and the optimum absorption temperature is close to 300xc2x0 C. These values can constitute a considerable technological complication in using said materials. Furthermore, the high relative density of the compound, caused by the presence of a relatively heavy ion such as Ba, reduces the effectiveness of the absorbent, which can be expressed as the mass of NOx, absorbed to saturation per gram of absorbent.
The aim of the present invention is to eliminate the above-mentioned drawbacks in various known processes for absorbing nitrogen oxides from gas mixtures which comprise said oxides by devising a process which allows to absorb nitrogen oxides at lower temperatures with equal efficiency.
An object of the present invention is to provide a nitrogen oxide absorption process in which the regeneration temperature of the absorbent is lower, accordingly ensuring an important technological advantage.
Another object of the present invention is to provide a process for absorbing nitrogen oxides at ambient temperature, albeit with reduced kinetics.
Another object of the present invention is to provide a process for absorbing nitrogen oxides which uses absorbent materials which are resistant to carbonatation if exposed to atmospheres similar to those of flue gas ducts (10% CO2, 10% H2O).
This aim, these objects and others are achieved by the process according to the present invention, which comprises the step of placing a gas mixture containing nitrogen oxides in contact with absorbent compounds comprising or constituted by mixed copper oxides chosen among CaCuO2, Sr14Cu24O41 derivatives thereof obtained by isovalent and/or heterovalent substitutions, and mixtures thereof. Advantageously, the CaCuO2 and Sr14Cu24O41 derivatives are derived by isovalent and/or heterovalent substitutions both on the sites occupied by the metals of the second group (for example Sr or Mg on Ca in CaCuO2 and Ca, La, Na on Sr in Sr14Cu24O41) and on the sites occupied by copper (for example fourth-period transition metals such as Ni).
Selective NO and NO2 absorption properties have been found unexpectedly in CaCuO2 and Sr14Cu24O41 compounds and in compounds derived therefrom by isovalent and heterovalent substitutions on the sites occupied by metals of the second group.
Both compounds and their derivatives obtained by substitution are known in the literature (for CaCuO2, Roth et al., J Am Ceram Soc, Vol. 72, p. 1545 (1989), JCPDS card no. 46-0054; for Sr14Cu24O41, McCarron et al., Mat Res Bull, Vol. 23, p. 1355 (1988), JCPDS card no. 43-0025); in particular, for the Sr14Cu24O41 compound there is a considerable body of literature associated with its unusual magnetic properties.
Hereinafter, when referring to the CaCuO2 and Sr14Cu24O41 compounds, such reference is to be understood as a reference to compounds which produce powder diffraction spectra corresponding to JCPDS card no. 46-0054 for CaCuO2 and, JCPDS card no. 43-0025for Sr14Cu24O41, respectively.
However, no reference to the absorption properties of these materials or to their use in any application thereof has been noted.